RecruitingBehavioural intervention7-Days Water-Only Fasting or Ketone Supplementation Trial in Prostate CancerActivating the immune system to kill cancer cells is a promising therapeutic strategy for some patients with cancer. Unfortunately, current immune-targeting treatments do not work for patients with prostate cancer. In animal models of cancer, fasting can reprogram the body's metabolism and immune cell function to help immune cells kill cancer cells likely by increasing ketosis. The purpose of this study is to determine whether 7 days of water-only fasting or a ketogenic very low-calorie diet or ketone supplementation is safe and feasible for patients with prostate cancer. This will help the investigators develop future studies to test whether incorporating periods of fasting or very low calorie diets or a ketone supplement into treatment plans for prostate cancer can improve outcomes from this disease.